NOTE: Not all of these reasons may apply to you (3&4) but I put them there just in case :). Thanks for debating!
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Capitalism and communism stand at opposite poles. Their essential difference is this: The communist, seeing the rich man and his fine home, says: 'No man should have so much.' The capitalist, seeing the same thing, says: 'All men should have as much.'
-- Phelps Adams

The great virtue of a free market system is that it does not care what color people are; it does not care what their religion is; it only cares whether they can produce something you want to buy. It is the most effective system we have discovered to enable people who hate one another to deal with one another and help one another. -- Milton Friedman

In a few words, the chief reason communism cannot work is free will and human nature (although there are more reasons). The definitions relevant to this paper are listed below. All are taken from the Merriam-Webster Dictionary .

FREE WILL:
1 : voluntary choice or decision "I do this of my own free will."
2 : freedom of humans to make choices that are not determined by prior causes or by divine intervention.

HUMAN NATURE:
the nature of humans; especially : the fundamental dispositions and traits of humans "It is human nature to sin."

I believe human nature consists of three main principles: The desire to achieve, individuality, and greed/envy. The importance of these two phrases will be made clear in the next section.

Reason it Cannot Work No. 1: Free Will & Human Nature

The communist ideology fails to take into account free will and human nature. In a communist ideal, all will work together as a well-oiled and productive machine. Of course, we see things such as this every day. Termites maintain their nests day in and day out. They work with great efficiency day in and day out, quite literally for the good of the mound.

This is not how humans function. You may disagree with my idea of human nature, but you cannot deny that every human being has at least a hint of self-respect and free will. Termites do not. It is because of this, we can't act like perfect, harmonious, machines.

Communism expects each and every one of the people to work together and to be willing to sacrifice for the good of the people. Communism requires that each and every worker lose their free will and affinity -- for with these, comes the desire to achieve above the rest, which is quite contradictory to standard communist ideals. It quite literally contradicts the ideas of nature and free will. It brazenly ignores these, and pretends that all are willing to sacrifice themselves, for the collective.

Since humans have free will, there cannot be a total communist state (in a popular majority as required by Marx), and therefore, it is an invalid way of doing things. There will always be rebels, those who refuse to work for the good of the community, and the larger that community is the larger the chance of those rebels. Thus, not every nation can be "truly" communist, and therefore, the "utopia" that communism seeks to attain is, alas, unattainable with more than about 1,000 people.

RICW No. 2: The Economic Issues

Say I have communist community that out of ‘Z' people total, there are 100 adult, able-bodied people. Twenty of us each work at a clothing factory, a food processor/farm, a housing builder, and medical care center/pharmaceuticals institute, and a police/fire station. We all manage to do our jobs and distribute our goods/services for the good of the community. With these five services/products, the people in Village X can live with a merely "OK" standard of living at best. They've got food, clothing, healthcare, housing, and protection. This all costs ‘Y' amount of manpower/labor/money per year.

Now, let us say I want to raise the standard of living significantly. Let us also say that I can create all of these products in one factory with twenty people (even though this isn't the case -- some would require less, like the chair, and some would require many more, like the car). I would like to add to each household:
A car
A TV
Heating/Cooling systems
Indoor plumbing
Electricity
Now, in order to attain these, I would have to create more factories, laborers, etc. This would take a lot of time and effort, just for these five improvements, and you'd have to add each of the five improvements to, say, the fifty houses in my village.

So, in order to have the population live in a great standard of living, I'd need more of ‘Y', but with more of ‘Y' comes more ‘Z', which in turn requires more ‘Y' than it gives. It becomes endless loop that can only provide its people with the most basic of needs at best. With each new family comes a new house, which I'd have to give them the ten products/services, and the family becomes a giant collective debt machine -- they require more than they give for their labor input.

Thus, purely communist societies cannot enjoy a very good standard of living because of the severe limitations it involves, especially the larger ones.

RICW No. 3: "Fairness" and Natural Selection

I myself believe in evolution. If you don't, then this section probably doesn't have a lot of weight. I could explain why I believe in evolution, but that's another topic for another paper.

One of the main principles of Charles Darwin's Theory of Evolution is called natural selection. Natural selection is basically the that the strongest of the species live on and the weakest die out. Economically, communism works against this in that it seeks to balance out the community in the most extreme sense, eliminate failure, and make society "fair."

Which brings me to fairness. The freedom to fail is perhaps one of the most important in a democratic society. While communism seeks to cushion failure (which is ultimately counter-productive, because failure breeds success and new inventions, ideas, and thoughts) capitalism embraces it as a natural part of any person's, economy's, or government's being.

You cannot be a communist, who believes in the good of the community, and a believer of evolution as a natural law -- for it concerns the surviving of the individual (NOT the survival of species in general).

RICW No. 4: Universal Requirements

Karl Marx clearly stated in his Manifesto that in order for communism to work, the whole globe would have to be one big collective. I've already touched upon how this is possible in RICW No. 2, but this is specifically to address those who say the Warsaw Pact would eventually become global and achieve the Marxist dream.

The seven non-Russian states were chiefly satellite states; the Soviets supported smaller states like Hungary, Poland, or Albania in a way a puppet-master props up his dolls. For instance, the Soviet-backed Komunistick� Strana Československa (Czechoslovakia Communist Party) overthrew the government in Czechoslovakia via coup d'�tat. The story is the same for most nations, such as Latvia, Belarus, or Lithuania. Insinuating that a utopia would be achieved with the Warsaw Pact is absurd; most of the nations joined forcibly. Therefore, the Warsaw Pact was not a harmonious alliance that was near creating a communist paradise on earth.

Recap"

1: It works against human nature, whereas capitalism utilizes it to better the individual who works. In almost every instance of a wealthy person, we have someone who somewhere along the line, worked hard to get their fortunes. (Except for lotteries. But who wins those?)

2: The standard of living in communism would be quite low, especially considering the population of the world. If all 6 billion+ of us were communist, our standard of living would be quite low, especially in comparison to the average capitalists'.

3: It violates the theories of Natural Selection/Evolution. If you believe in these two things, you cannot be communist.

I'm not a true Stalinist or Marxist, the only reason I put that as my Party/Ideology is because I support a resource-based economy, which is the closest thing to it. I suggest debating with a true communist instead of me, but unless you still want to debate, I will say this: Greed is not the cause for a capitalistic/monetary system, it is the other way around. No one is born with greed/prejudice etc, they are the byproducts of the environment in which they grew up in. If a system glorifies competition and greed, then people will thrive to be competitive and greedy. If a system requires the cooperation of people, then people will thrive to be cooperative. In a sense, there is no such thing as 'human nature,' as it would be impossible from everyone to be the same. I advocate for a system in which labor is automated, and the products from automated labor are equally available to everyone. Car factories are mostly automated, and currently about 90% of jobs have the potential to be automated. Capitalism is collapsing and becoming too outdated for our time. Once more and more companies start automating jobs because of their efficiency from lack of human error, lack of the need for breaks, and cost efficiency, then we will need to get a new economic system, such as a resource-based economy.

//Greed is not the cause for a capitalistic/monetary system, it is the other way around. No one is born with greed/prejudice etc, they are the byproducts of the environment in which they grew up in. If a system glorifies competition and greed, then people will thrive to be competitive and greedy. If a system requires the cooperation of people, then people will thrive to be cooperative. In a sense, there is no such thing as 'human nature,' as it would be impossible from everyone to be the same.//

If you know anything about toddlers, you should have come to a different conclusion. Many toddlers are very selfish, and they are often raised by people who encourage their children to share. Yet, though, there are still those who won't share their toys, because they are greedy. It's not the system that shapes them (safe to say most toddlers don't understand economics)This, in my opinion, is a significant argument against your Tabula Rasa ideas. I'm sure you can think of other examples, like warring tribes in Africa. Tribes are basically collectives, but at the same time, have conflict, which often occurs because of territorial disputes -- someone wants more.

//Capitalism is collapsing and becoming too outdated for our time. Once more and more companies start automating jobs because of their efficiency from lack of human error, lack of the need for breaks, and cost efficiency, then we will need to get a new economic system, such as a resource-based economy.//

From the sounds of this, it seems like Capitalism is the predecessor to a resource-based economy. Why, then, do you not support it? Can Communism reach the same end?